Bubbles in immersion lithography

One of the major concerns in the emerging technology of liquid immersion lithography is the possibility of defects due to bubble formation in the immersion liquid. We have performed numerical simulations of the impact of bubbles on printing. The greatest impact is due to bubbles close to the resist surface; beyond several bubble diameters, the impact of a bubble is negligible. With this in mind, we have investigated one of the major candidates for producing near-surface bubbles, resist outgassing. We have imaged, with 0.25μm∕pixel resolution, the surface of an immersed resist immediately after exposure to 193nm radiation at a fluence of 2–20mJ∕cm2. Using a high outgassing 248nm resist, bubble formation is clear. However, in over 3000 frames of a model 193nm immersion resist, only one frame with bubble candidates was seen, and only at a fluence (20mJcm−2) far exceeding that of normal exposure conditions.